1. Field of the Invention
The present invention relates to a digital tape measure in which a tape is stored in a casing body and is movable being drawn in and out through an aperture of the casing body, the amount of movement of the tape is detected by photoelectric detecting means, and the detected signal is processed so that the measure length may be digitally displayed.
2. Prior Art
Generally, when a detection mark for measuring length given on the tape at equivalent interval is detected by means of photoelectric transfer (conversion) element, it is necessary for the tape to pass through a portion which is distant from the photoelectric transfer element by a length corresponding to the focal length of the photoelectric transfer element. This is because the quantity of reflecting light given to the photoelectric transfer element is insufficient when the distance between the photoelectric element and the tape is too large, while the output wave from the photoelectric element is influenced by the diffraction when the distance is too small, and in both cases the detection mark for measuring length cannot be exactly read.
Accordingly, it is conventional to hold the tape between compression rollers so that the tape is prevented from oscillation when it is drawn out. In particular, in order to ensure the exact movement of the tape keeping the focal distance, a method was already proposed wherein a guide plate is provided at the position distant from the photoelectric transfer element by a length corresponding to the focal length, and a tape is held between the rollers at the positions before and behind the guide plate so that the tape may be moved while sliding on the guide plate, as is exemplified in Japanese Utility Model Publication (unexamined) No. 55-130206.
However, in case that the tape is compressively held between the rollers as mentioned above, unless the rolling friction of the rollers is small, it is difficult to rewind and accept the tape into the casing by using the elasticity of the rewinding spring connected to the inner end of the casing. Also in case that the tape is slid on the guide plate while being held between the rollers, the movement of the tape meets with resistance.
The compression of rollers to the tape brings about stain or peeling of the detection mark for measuring length given on the surface of the tape, and sometimes deterioration of reflection efficiency is caused thereby.
In case that the tape is made of steel and of which cross section is circular arc so as to keep linear by the proper elasticity when drawn out to a certain length, since the friction is increased when the tape is compressively held by the rollers, the above-described problem is particularly serious.